U.S. patent application number 13/702973 was filed with the patent office on 2013-04-04 for apparatus for euv imaging and methods of using same.
This patent application is currently assigned to KLA-TENCOR CORPORATION. The applicant listed for this patent is Damon F. Kvamme, James P. McGuire, JR., John M. Rodgers, John R. Rogers, Daniel C. Wack. Invention is credited to Damon F. Kvamme, James P. McGuire, JR., John M. Rodgers, John R. Rogers, Daniel C. Wack.
Application Number | 20130083321 13/702973 |
Document ID | / |
Family ID | 46507622 |
Filed Date | 2013-04-04 |
United States Patent
Application |
20130083321 |
Kind Code |
A1 |
Wack; Daniel C. ; et
al. |
April 4, 2013 |
APPARATUS FOR EUV IMAGING AND METHODS OF USING SAME
Abstract
One embodiment relates to an apparatus that includes an
illumination source (102) for illuminating a target substrate
(106), objective optics (108) for projecting the EUV light which is
reflected from the target substrate, and a sensor (110) for
detecting the projected EUV light. The objective optics includes a
first mirror (202,302, or 402) which is arranged to receive and
reflect the EUV light which is reflected from the target substrate,
a second mirror (204, 304, or 404) which is arranged to receive and
reflect the EUV light which is reflected by the first mirror, a
third mirror (206, 306, or 406) which is arranged to receive and
reflect the EUV light which is reflected by the second mirror, and
a fourth mirror (208, 308, or 408) which is arranged to receive and
reflect the EUV light which is reflected by the third mirror.
Inventors: |
Wack; Daniel C.;
(Fredericksburg, VA) ; Kvamme; Damon F.; (Los
Gatos, CA) ; Rogers; John R.; (Monrovia, CA) ;
McGuire, JR.; James P.; (Pasadena, CA) ; Rodgers;
John M.; (Pasadena, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Wack; Daniel C.
Kvamme; Damon F.
Rogers; John R.
McGuire, JR.; James P.
Rodgers; John M. |
Fredericksburg
Los Gatos
Monrovia
Pasadena
Pasadena |
VA
CA
CA
CA
CA |
US
US
US
US
US |
|
|
Assignee: |
KLA-TENCOR CORPORATION
MILPITAS
CA
|
Family ID: |
46507622 |
Appl. No.: |
13/702973 |
Filed: |
January 6, 2012 |
PCT Filed: |
January 6, 2012 |
PCT NO: |
PCT/US12/20504 |
371 Date: |
December 7, 2012 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
61431768 |
Jan 11, 2011 |
|
|
|
Current U.S.
Class: |
356/239.3 ;
359/838 |
Current CPC
Class: |
G03F 1/84 20130101; G03F
1/24 20130101; B82Y 10/00 20130101; B82Y 40/00 20130101 |
Class at
Publication: |
356/239.3 ;
359/838 |
International
Class: |
G03F 1/84 20060101
G03F001/84 |
Claims
1. An apparatus for inspecting a photomask using extreme
ultra-violet (EUV) light, the apparatus comprising: an illumination
source for generating the EUV light which illuminates a target
substrate; objective optics for receiving and projecting the EUV
light which is reflected from the target substrate; and a sensor
for detecting the EUV light which is projected by the objective
optics, wherein the objective optics comprises a first mirror which
is arranged to receive and reflect the EUV light which is reflected
from the target substrate, a second mirror which is arranged to
receive and reflect the EUV light which is reflected by the first
mirror, a third mirror which is arranged to receive and reflect the
EUV light which is reflected by the second mirror, and a fourth
mirror which is arranged to receive and reflect the EUV light which
is reflected by the third mirror.
2. The apparatus of claim 1, wherein the second mirror partially
obscures the first mirror from the reflected EUV light.
3. The apparatus of claim 1, wherein the light reflected by the
second mirror passes through an opening in the first mirror.
4. The apparatus of claim 1, wherein the first, second, third, and
fourth mirrors are, respectively, concave, concave, convex and
concave.
5. The apparatus of claim 1, wherein the first, second, third, and
fourth mirrors are, respectively, concave, convex, concave and
convex.
6. The apparatus of claim 1, wherein the first, second, third, and
fourth mirrors are, respectively, concave, convex, concave, and
concave
7. The apparatus of claim 1, wherein the numerical aperture of the
objective optics is greater than 0.2.
8. The apparatus of claim 1, wherein a field of view of the
apparatus is at least greater than 5,000 square microns.
9. The apparatus of claim 1, wherein a distance between the target
substrate and the second mirror is greater than 100
millimeters.
10. Objective optics for extreme ultra-violet light formed by an
arrangement of mirrors, the objective optics comprising: a first
mirror which is arranged to receive and reflect the EUV light which
is reflected from the target substrate, a second mirror which is
arranged to receive and reflect the EUV light which is reflected by
the first mirror, a third mirror which is arranged to receive and
reflect the EUV light which is reflected by the second mirror, and
a fourth mirror which is arranged to receive and reflect the EUV
light which is reflected by the third mirror, wherein the numerical
aperture of the objective optics is greater than 0.2.
11. The objective optics of claim 10, wherein the second mirror
partially obscures the first mirror from the reflected EUV
light.
12. The objective optics of claim 10, wherein the light reflected
by the second mirror passes through an opening in the first
mirror.
13. The objective optics of claim 10, wherein the first, second,
third, and fourth mirrors are, respectively, concave, concave,
convex and concave.
14. The objective optics of claim 10, wherein the first, second,
third, and fourth mirrors are, respectively, concave, convex,
concave and convex.
15. The objective optics of claim 10, wherein the first, second,
third, and fourth mirrors are, respectively, concave, convex,
concave, and concave
16. The objective optics of claim 10, wherein a field of view of
the apparatus is at least greater than 5,000 square microns.
17. The objective optics of claim 10, wherein a distance between
the target substrate and the second mirror is greater than 100
millimeters.
18. A method of projecting extreme-ultraviolet light reflected from
a manufactured substrate to a sensor, the method comprising:
receiving and reflecting the EUV light which is reflected from the
target substrate by a first mirror; receiving and reflecting the
EUV light which is reflected from the first mirror by a second
mirror; receiving and reflecting the EUV light which is reflected
from the second mirror by a third mirror; receiving and reflecting
the EUV light which is reflected from the third mirror by a fourth
mirror; and detecting the EUV light which is reflected by the
fourth mirror.
19. The method of claim 18, wherein the numerical aperture of the
objective optics is greater than 0.2.
20. The method of claim 18, wherein the second mirror partially
obscures the first mirror from the reflected EUV light.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] The present application claims the benefit of U.S.
provisional patent application No. 61/431,768, entitled "Apparatus
for EUV Imaging and Methods of Using Same," filed Jan. 11, 2011,
the disclosure of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Technical Field
[0003] The present disclosure relates to optical apparatus and
methods of using same.
[0004] 2. Description of the Background Art
[0005] The conventional apparatus in the market for photomask
inspection generally employ ultra-violet (UV) light with
wavelengths at or above 193 nanometers (nm). This is suitable for
masks designed for use in lithography based on 193 nm light. To
improve further the printing of minimum feature sizes, next
generation lithographic equipment is now designed for operation in
the neighborhood of 13.5 nm. Accordingly, patterned masks designed
for operation near 13 nm must be inspected. Such masks are
reflective, having a patterned absorber layer over a
resonantly-reflecting substrate (EUV multilayer, typically 40 pairs
of MoSi with a 7 nm period. The conventional inspection apparatus
uses optics with a combination of wavelength and numerical
apertures (NA) that are not sufficient (i.e. too small) to resolve
pattern features and pattern defects of interest (printable) in EUV
mask patterns characterized by a half-pitch below 22 nanometers
(nm).
SUMMARY
[0006] One embodiment disclosed relates to an apparatus for
inspecting a photomask using extreme ultra-violet (EUV) light. The
apparatus includes an illumination source for generating the EUV
light which illuminates a target substrate, objective optics for
receiving and projecting the EUV light which is reflected from the
target substrate, and a sensor for detecting the EUV light which is
projected by the objective optics. The objective optics includes a
first mirror which is arranged to receive and reflect the EUV light
which is reflected from the target substrate, a second mirror which
is arranged to receive and reflect the EUV light which is reflected
by the first mirror, a third mirror which is arranged to receive
and reflect the EUV light which is reflected by the second mirror,
and a fourth mirror which is arranged to receive and reflect the
EUV light which is reflected by the third mirror.
[0007] Other embodiments, aspects and features are also
disclosed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is a schematic diagram of a reflective imaging
apparatus in accordance with an embodiment of the invention.
[0009] FIG. 2 is an optical ray diagram of a mirror distribution
for reflective objective optics in accordance with a first
embodiment of the invention.
[0010] FIG. 3 is an optical ray diagram of a mirror distribution
for reflective objective optics in accordance with a second
embodiment of the invention.
[0011] FIG. 4 is an optical ray diagram of a mirror distribution
for reflective objective optics in accordance with a third
embodiment of the invention.
DETAILED DESCRIPTION
[0012] EUV microscope objectives (having 2 to 4 multilayer-coated
mirrors) disclosed previously, designed for defect or pattern
review applications, with operation in the neighborhood of 13 nm
wavelength of light, are based have NA in the range 0.05-0.12, and
an object field extent just adequate to the review (microscopy)
task--in the range of 5-20 microns at the mask. According to
rigorous analyses of the defect detection capability as a function
of NA and defect size, the resolving power and defect detection
sensitivity of EUV objectives with NA in this range are inadequate
for production-worthy EUV mask inspection of masks with feature
half-pitch (HP) below 18 nm or so, due to shortcomings in both NA
and high-quality object field size.
[0013] Current mask inspection systems are based on UV and DUV
laser sources of light, which are high brightness and relatively
high power. Light sources with significant spectral brightness in
the neighborhood of 13 nm are based on pulsed plasmas, with
temperatures in the range 20-50 eV. Due to poor conversion
efficiency (conversion from input energy to in-band radiation),
such plasma sources show limited brightness at 13-14 nm, and
raising the brightness significantly can drive source cost (and
thus inspection costs imposed on the mask during fabrication) to
levels which impair the economic attractiveness of EUV Lithography
(EUVL).
[0014] High-throughput operation of mask inspection systems with
low brightness plasma sources (discharge or laser produced) drives
the need for large object field and detector array, to increase the
rate of instantaneous image signal integration and conversion to
digital representation.
[0015] Simultaneously, to discriminate defect signals from
background image noise, the imaging optics must maximize the
collection of light diffracted or scattered by patterning or
multilayer defects residing on the EUV mask of interest. For most
defects of interest, which diffract and scatter the incident light
over a wide range of angles, increasing the NA of the objective
will provide an increase in defect signals.
[0016] Multilayer-mirror based imaging systems have poor
transmission of light, due to the limited reflectivity of
multilayers at the design wavelengths near 13-14 nm. A single MoSi
multilayer mirror shows peak spectral reflectivity near 13.5 nm in
the range of 60-70%. After multiple reflections from near-normal
incidence mirrors in typical illumination and imaging optics in an
EUV system, system transmission can fall below 1%.
[0017] To perform the inspection task adequately, the light
reaching the image plane and converted to digital signals by the
detector array, from each resolved region of the mask, must reach a
certain number of primary (13 nm) quanta, and so a certain minimum
signal-to-noise ratio, which in well designed systems is a strong
function of the number of primary quanta (photons absorbed in the
detector material, typically silicon). To compensate for losses in
the optical system while keeping the light incident on the detector
constant, the source brightness must be increased, which is
difficult to develop and expensive to produce using currently known
source technologies.
[0018] Alternatively, the range of angles emitted by the source
which are transferred to the mask by the illumination optics can be
increased, since the amount of light will increase with this
angular range, at least within a range of angles supported by the
source brightness. In other words, the illumination pupil size can
be increased until a physical constraint intervenes. Rigorous
studies of defect SNR in inspection optic designs have indicated
that for EUV masks, such largely incoherent imaging often provides
higher SNR than lower sigma, more coherent operation of the design
and system, when used with plasma sources of limited
brightness.
[0019] The use of beam splitters in reflective imaging systems used
in conjunction with reflective objects (such as EUV mask inspection
using EUV light) can simplify optical design and layout, by
allowing interpenetration or overlap of illumination and imaging
pupils in angle space. Current EUV beam splitter technology have
low reflection and transmission coefficients (25-35%). Inspection
systems with beamsplitters must increase source brightness greatly
to compensate for the loss of light reaching the detector.
Inspection optics without a beamsplitter element is thus strongly
preferred.
[0020] Light at wavelengths within the spectral bandpass of the
resonantly-reflecting multilayer incident on such a uniform
(unpatterned) mirror is reflected at 60-70% only if the angle of
incidence resides within the angular bandpass as well. Periodic
MoSi multilayers have an angular bandpass of 20-25 degrees at 13.5
nm. Light incident outside of the angular bandpass is reflected by
the multilayer at very low levels, and thus is largely absorbed, or
wasted.
[0021] Rigorous studies of light propagation and diffraction by
patterns on EUV masks indicates that this trend holds for light
incident on patterned masks, as well. Furthermore, the angular
distribution of light diffracted and scattered by defects present
on or in the EUV patterned mask is also modulated by the angular
bandpass of the multilayer. The angular distribution of light
scattered by a defect depends as well on the defect geometry, and
the geometry of the local pattern, and can be significantly skewed
to one side of the imaging pupil or another. To collect adequate
light from all defect types and for arbitrary pattern geometries,
the size of the imaging pupil should be maximized. Consequently,
design of inspection optics without a beamsplitter and which
operate largely within the finite angular bandwidth of the mask,
and which utilize plasma sources of limited brightness, must
contend with competing angular claims of the illumination and
imaging pupils, each of which seek to maximize the size of their
angular extent.
[0022] EUVL at 11 HP may use aperiodic multilayers in the EUV mask
design, which provide increased angular bandwidth, and enable EUVL
imaging at higher NAs than possible with a conventional periodic
multilayer design. This improves, but does not fully mitigate the
issue of finite angular bandpass.
[0023] Although increasing the number of mirrors in an imaging
design can provide design capability which enables simultaneous
high NA and wide object field, this can lead to a prohibitive
decrease in light reaching the detector. Thus, there is significant
value in discovering designs which provide adequate inspection
performance at minimum mirror count, which do not use a beam
splitter, and which balance the competing needs of illuminating and
imaging pupils sizes and locations, and thereby enable the
production use of low brightness plasma-based EUV sources.
[0024] Furthermore, it is of strong economic interest to discover
optical designs which provide adequate defect inspection
performance for at least two technology nodes, for example 16 HP
and 11 HP. As the critical defect size which limits chip yield
shrinks with technology node, the NA of the inspection system
should be increased to compensate for the reduction in scattered
light.
[0025] In summary, conventional apparatus that utilize UV light are
clearly limited when applied to inspect extreme ultra-violet
photomasks ("EUV masks"). The conventional apparatus are typically
"non-actinic" in that they result in an image of the mask that does
not represent what will be realized using EUV light during
lithography. Rather, the resultant image of the mask lacks both
resolution and contrast, such that the image is not very useful for
pattern inspection and defect detection.
[0026] More recently, photomask inspectors that use EUV light for
imaging ("EUV inspectors") are being developed. However, the
current EUV inspectors also have limitations and drawbacks. First,
the field sizes of the images are very small. This limitation
results in a low throughput when the apparatus is used to inspect
entire EUV masks for defects. Second, numerical apertures of the
optics are low. This limitation results in a relatively lower
resolution. Such lower resolution limits the practical use of the
images for pattern inspection and defect detection.
[0027] The present patent application discloses reflective imaging
apparatus that overcome the above-discussed problems with photomask
inspectors.
[0028] During inspection of patterned masks, acquisition and
subsequent signal processing of the signal corresponding to a
localized defective pattern can be accomplished by comparing or
differencing the digital images from a test region of a pattern and
a reference region, whether acquired or synthesized from prior
information. Such difference operation removes the pattern, leaving
the defect as a perturbation of a quasi-uniform background
signal.
[0029] Imaging pupils are often circularly symmetric, leading to
symmetric point spread functions at the image plane. While such
symmetry is often required in lithography, mask inspection via
difference imaging does not require symmetric psf, and consequently
the imaging pupil can afford to be asymmetric. In particular,
obscuration of a portion of the imaging pupil can be tolerated, if
defect signal collection is not compromised significantly.
[0030] Additionally, the shape of the parent pupil need not be
circular. For instance, square or rectangular shapes for the parent
are possible, and even advantageous when considering the
incremental gain of scattered defect light or signal through
addition of pupil region.
[0031] Expressed as a fraction of pupil area, obscuration fractions
less than 5 or 10% are preferred. Obscuration in 4-mirror designs
is often created through the blocking or shadowing of light
reflected or scattered from the mask by the second mirror, or M2.
Minimizing the size of both reflecting surface and peripheral
support of M2 will minimize obscuration.
[0032] The design of structural support for M2 must provide for
sufficient rigidity, so that environmental disturbances or
vibrations do not drive or lead to dynamic perturbations of M2
position and thus to degradation of image quality through
blurring.
[0033] Since mirrors for EUV light must be coated with multilayers
to reach adequate reflectivity, the range of incidence angles on
any of the highly curved elements must be considered, and
restricted within the limits of multilayer deposition process
technology. When estimating the defect SNR of a particular
objective and system design, the apodization or modulation of
transmission of each light ray by local reflectivity variations at
the point of reflection on each mirror induced by the multilayer
deposition process must be considered.
[0034] In particular, the design process must balance obscuration,
structural response and curvature factors in the geometry of the
second mirror or M2, in order to secure the minimum viable defect
SNR which enables fast and economic mask inspection.
[0035] The choice of chief ray in design of the objective for mask
inspection must balance several competing factors. The chief ray is
defined by the centroid of the angular distribution of light rays
transmitted by the objective to the image plane; i.e., with due
consideration of the pupil apodization caused by mirror coatings.
Although conventional designs for reflective imaging without a
beamsplitter place the plane dividing the illumination and
collection light bundles on the optical axis and coincident with
the object surface normal, inspection-oriented optics do not demand
or strongly prefer this choice. Thus allowing placement of the
lower marginal ray of the imaging pupil below the surface normal is
found to be advantageous for defect signal collection.
[0036] Correspondingly, in the process of increasing defect SNR, as
the NA is increased from low levels, in higher performance designs
the imaging chief ray (relative to the surface normal) is below the
numerical value of the NA. Inspection-optimized EUV objective
designs bias the imaging chief rays toward the surface normal to
maximize overlap of imaging pupil with multi-layer modulated
angular distribution of light scattered by pattern defects, while
providing sufficient angular range (still largely restricted to the
multilayer angular bandpass) to the illumination pupil to secure
adequate photon flux from the limited brightness plasma EUV
sources.
[0037] FIG. 1 is a schematic diagram of a reflective imaging
apparatus in accordance with an embodiment of the invention. The
apparatus 100 includes an EUV illumination source 102, an
illumination mirror 104, a target substrate 106, a substrate holder
107, objective optics 108, a sensor (detector) 110, and a data
processing system 112.
[0038] The EUV illumination source 102 may comprise, for example, a
laser-induced plasma source which outputs an EUV light beam 122. In
one embodiment, the EUV light is at a wavelength of 13.5 nm. The
illumination mirror 104 reflects the EUV light such that the beam
124 illuminates the target substrate 106. In one embodiment of the
invention, the target substrate 106 is an EUV mask being
inspection. The target substrate 106 may be scanned under the beam
124 by controllably translating the substrate holder 107 so that
the field of view of the imaging apparatus covers regions on the
substrate to be inspected.
[0039] Patterned light 126 is reflected from the target substrate
106 to the reflective objective optics 108. Preferred embodiments
of the objective optics 108 are described in detail below in
relation to FIGS. 2, 3 and 4.
[0040] The objective optics 108 outputs a projection 128 of the
patterned light onto the sensor 110. In one embodiment, the sensor
110 may be a time-delay integration detector array so that the data
may be detected while the target substrate is being scanned
(translated) under the beam 124.
[0041] The data processing system 112 may include electronic
circuitry, one or more microprocessors, data storage, memory and
input and output devices. The data processing system 112 may be
configured to receive and process data from the sensor 110. In
accordance with one embodiment, the data processing system 112 may
process and analyze the detected data for pattern inspection and
defect detection.
[0042] FIG. 2 is an optical ray diagram of a mirror distribution
for the objective optics 108 in accordance with a first embodiment
of the invention. An optical prescription for the objective optics
108 in FIG. 2 is provided below in Appendix A.
[0043] In this embodiment, there are four mirrors (202, 204, 206,
and 208) arranged as shown in FIG. 2. The mirrors are arranged such
that the patterned light 126 reflects from the first, second,
third, and fourth mirrors (202, 204, 206, and 208, respectively) in
that order. In this arrangement, the first mirror 202 is concave,
the second mirror 204 is concave, the third mirror 206 is convex,
the fourth mirror 208 is concave. Hence, the mirrors are, in order:
concave; concave; convex; and concave.
[0044] In this embodiment, the second mirror 204 partially obscures
the first mirror 202 from the patterned light 126. In other words,
part of the area of the first mirror 202 is blocked by the second
mirror 204 from receiving the light 126 reflected from the target
substrate 106. Furthermore, an opening in the first mirror 202 is
used to let the light reflected by the second mirror 204 pass
through to reach the third mirror 206. Applicants have determined
that, despite the first mirror 202 being partially obscured and
needing a pass-through hole, a high numerical aperture is
nevertheless achieved with this embodiment.
[0045] In accordance with a preferred embodiment, the numerical
aperture for the objective optics is at least 0.2, and the field of
view is at least 5,000 square microns in area. For this
implementation of the objective optics 108, the numerical aperture
has been determined to be 0.24, and the size of the field of view
has been determined to be 327 microns by 30 microns (9,810 square
microns in area). Advantageously, both the numerical aperture and
field of view are relatively large in this embodiment.
[0046] The working distance is the distance between the target
substrate 106 and the nearest optical element (in this case, the
second mirror 204). A working distance of at least 100 millimeters
(mm) is desirable to provide sufficient space for illumination of
the target substrate 106. In this embodiment, the working distance
is 145 mm.
[0047] The total track may be defined as the distance from the
target substrate 106 to the sensor 110. In this particular
embodiment, the total trace is 1,500 mm.
[0048] FIG. 3 is an optical ray diagram of a mirror distribution
for reflective objective optics in accordance with a second
embodiment of the invention. An optical prescription for the
objective optics 108 in FIG. 3 is provided below in Appendix B.
[0049] In this embodiment, there are four mirrors (302, 304, 306,
and 308) arranged as shown in FIG. 3. The mirrors are arranged such
that the patterned light 126 reflects from the first, second,
third, and fourth mirrors (302, 304, 306, and 308, respectively) in
that order. In this arrangement, the first mirror 302 is concave,
the second mirror 304 is concave, the third mirror 306 is convex,
the fourth mirror 308 is concave. Hence, the mirrors are, in order:
concave; convex; concave; and convex.
[0050] In this embodiment, the second mirror 304 partially obscures
the first mirror 302 from the patterned light 126. In other words,
part of the area of the first mirror 302 is blocked by the second
mirror 304 from receiving the light 126 reflected from the target
substrate 106. Furthermore, an opening in the first mirror 302 is
used to let the light reflected by the second mirror 304 pass
through to reach the third mirror 306. Applicants have determined
that, despite the first mirror 302 being partially obscured and
needing a pass-through hole, a high numerical aperture is
nevertheless achieved with this embodiment.
[0051] In accordance with a preferred embodiment, the numerical
aperture for the objective optics is at least 0.2, and the field of
view is at least 5,000 square microns in area. For this
implementation of the objective optics 108, the numerical aperture
has been determined to be 0.24, and the size of the field of view
has been determined to be 440 microns by 420 microns (184,800
square microns in area). Advantageously, the numerical aperture is
relatively large in this embodiment, and the field of view is
particularly large. The large field of view advantageously enables
multiple sensor columns.
[0052] The working distance is the distance between the target
substrate 106 and the nearest optical element (in this case, the
second mirror 304). A working distance of at least 100 millimeters
(mm) is desirable to provide sufficient space for illumination of
the target substrate 106. In this embodiment, the working distance
is 237 mm.
[0053] The total track may be defined as the distance from the
target substrate 106 to the sensor 110. In this particular
embodiment, the total trace is 873 mm.
[0054] FIG. 4 is an optical ray diagram of a mirror distribution
for reflective objective optics in accordance with a third
embodiment of the invention. An optical prescription for the
objective optics 108 in FIG. 4 is provided below in Appendix C.
[0055] In this embodiment, there are four mirrors (402, 404, 406,
and 408) arranged as shown in FIG. 4. The mirrors are arranged such
that the patterned light 126 reflects from the first, second,
third, and fourth mirrors (402, 404, 406, and 408, respectively) in
that order. In this arrangement, the first mirror 402 is concave,
the second mirror 404 is convex, the third mirror 406 is concave,
the fourth mirror 408 is concave. Hence, the mirrors are, in order:
concave; convex; concave; and concave.
[0056] In this embodiment, the second mirror 404 partially obscures
the first mirror 402 from the patterned light 126. In other words,
part of the area of the first mirror 402 is blocked by the second
mirror 404 from receiving the light 126 reflected from the target
substrate 106. Furthermore, an opening in the first mirror 402 is
used to let the light reflected by the second mirror 404 pass
through to reach the third mirror 406. Applicants have determined
that, despite the first mirror 402 being partially obscured and
needing a pass-through hole, a high numerical aperture is
nevertheless achieved with this embodiment.
[0057] In accordance with a preferred embodiment, the numerical
aperture for the objective optics is at least 0.2, and the field of
view is at least 5,000 square microns in area. For this
implementation of the objective optics 108, the numerical aperture
has been determined to be 0.24, and the size of the field of view
has been determined to be 410 microns by 255 microns (104,550
square microns in area). Advantageously, the numerical aperture is
relatively large in this embodiment, and the field of view is also
large.
[0058] The working distance is the distance between the target
substrate 106 and the nearest optical element (in this case, the
second mirror 404). A working distance of at least 100 millimeters
(mm) is desirable to provide sufficient space for illumination of
the target substrate 106. In this embodiment, the working distance
is 230 mm.
[0059] The total track may be defined as the distance from the
target substrate 106 to the sensor 110. In this particular
embodiment, the total trace is 1,420 mm.
[0060] In the above description, numerous specific details are
given to provide a thorough understanding of embodiments of the
invention. However, the above description of illustrated
embodiments of the invention is not intended to be exhaustive or to
limit the invention to the precise forms disclosed. One skilled in
the relevant art will recognize that the invention can be practiced
without one or more of the specific details, or with other methods,
components, etc. In other instances, well-known structures or
operations are not shown or described in detail to avoid obscuring
aspects of the invention. While specific embodiments of, and
examples for, the invention are described herein for illustrative
purposes, various equivalent modifications are possible within the
scope of the invention, as those skilled in the relevant art will
recognize.
[0061] These modifications can be made to the invention in light of
the above detailed description. The terms used in the following
claims should not be construed to limit the invention to the
specific embodiments disclosed in the specification and the claims.
Rather, the scope of the invention is to be determined by the
following claims, which are to be construed in accordance with
established doctrines of claim interpretation.
* * * * *